A hydraulic buffer disclosed in Patent Literature (PTL) 1 includes a cylinder filled with pressurized hydraulic oil and a piston slidably disposed inside the cylinder so as to divide the cylinder into two oil chambers. A piston rod attached to the piston is led out of the cylinder, and the base portion of the cylinder is connected to a main body side via a mounting bracket, while the piston rod sticking out from the end portion of the cylinder is connected to an oscillating body side via another mounting bracket. The piston includes a passage for communication between the oil chambers and a pair of throttles and a pair of relief valves are provided in the passage, so as to apply attenuation force to the bi-directional flow of the hydraulic oil in the passage communicating between the oil chambers with the throttles and the valves in the piston, thus to attenuate the oscillation of the oscillating body.
The hydraulic buffer also includes an accumulator provided in a part of the space inside of the piston rod, to bias the accumulator piston with a spring to thereby pressurize the hydraulic oil, and the pressurization of the hydraulic oil is intended to increasing the volume elastic modulus of the hydraulic oil. To be more detailed, whereas the volume elastic modulus of the hydraulic oil decreases when air intrudes in the hydraulic oil, the volume elastic modulus is increased by increasing the pressure, and therefore a preload is applied to the hydraulic oil in the cylinder to allow the hydraulic buffer to exhibit a predetermined attenuation characteristic, despite a certain amount of air having intruded in the cylinder.
In the hydraulic buffer according to PTL 1, the pair of throttles and the pair of relief valves are provided in the piston. In addition a passage communicating with the accumulator has to be formed to install a pair of check valves, and further the accumulator and the pressure rod have to be inserted in the piston rod. Accordingly, the piston and the piston rod have to be significantly large in diameter and size, and the internal structure becomes highly complicated. Thus, although the intended purpose may be accomplished with the conceptual configuration shown in the drawings of PTL 1, it is extremely difficult to achieve a realistic design and therefore it is difficult to apply the configuration according to PTL 1 to small-sized fluid pressure cylinders widely employed.
The present inventor has proposed, as disclosed in PTL 2, a hydraulic shock absorber in which a piston chamber is filled in advance with the liquid including an amount to be consumed the chamber, so as to prolong the life span of the shock absorber. Basically, the shock absorber includes a piston provided in the piston chamber in a cylinder tube so as to move in the axial direction with a gap defined between the piston and the inner circumferential surface of the cylinder tube, and a moving object is made to collide with the end portion of a rod attached to the piston and sticking out from the cylinder tube, to thereby shock-absorbingly stop the moving object. The cylinder tube includes a reservoir tank surrounding the rod and in which an elastic member is provided, so that the liquid supplied from a supply hole of the cylinder tube can be stored in the reservoir tank at least with a preload applied to the liquid by the contraction of the elastic member.
Thus, in the shock absorber according to PTL 2 the elastic member provided in the reservoir tank secures a space, by the contraction of the elastic member, for accommodating the liquid flowing out of the piston chamber when the moving object collides with the rod, and the reservoir tank further serves as a chamber for additionally storing in advance the amount of liquid corresponding to the liquid leakage over a long-term use of the shock absorber. For such purpose, the supply hole for supplying the pressurized liquid into the liquid chamber is provided, so as to introduce the liquid subjected to the preload into the reservoir tank from the supply hole, and thus to effectively prolong the life span of the shock absorber.
As stated above, the shock absorber according to PTL 2 is configured to shock-absorbingly stop the moving object that has collided with the end portion of the rod connected to the piston and sticking out of an end portion of the cylinder tube. However, in general, the moving objects to be shock-absorbingly stopped by the shock absorber include a single moving object that reciprocates with respect to a fixed main body of an apparatus, and a pair of relatively moving objects that collide with each other while moving toward and away from each other either alternately or randomly, and in many cases both the reciprocal motion and the relative movement toward and away from each other have to be shock-absorbingly stopped. In such cases, anyway, a pair of shock absorbers capable of shock-absorbingly stopping the moving object against the moving direction of the object have to be provided. In the case where a bi-directional shock absorber having such a function is obtained, the bi-directional shock absorber is expected to be quite broadly applicable, by locating the shock absorber, directly or via a bracket if need be, between the moving object and the fixed main body of the apparatus supporting the moving object, or between the pair of relatively moving objects.
With the bi-directional shock absorber configured as above also, as with the shock absorber according to PTL 2, it is preferable to provide the reservoir tank in which the liquid subjected to the preload can be stored, to thereby effectively prolong the life span of the bi-directional shock absorber, however it is extremely difficult to provide such a bi-directional shock absorber of a highly simplified structure and in a reduced size, instead of merely providing a pair of shock absorbers in opposite directions.